1. Field of the Invention
The present invention relates to aqueous dispersions which comprise cellulose ethers in at least partly grafted form, processes for their preparation and the advantageous use of dispersions comprising these cellulose ethers for gluing, in particular for gluing polyolefins.
2. Description of Related Art
Aqueous copolymer dispersions based on vinyl acetate and ethylene are used in wide fields of the adhesives sector. The adhesives are optimized to a very high level above other adhesives in the packaging sector, in floor adhesives, in the furniture film field, and in many other areas of use. Although the adhesives available on the market to date meet the requirements regarding the desired adhesive force, measured as the shear and peel strength, on the usual materials, such as PVC, polyester, and polyurethanes, they do not do so on substitute materials for PVC, in particular polyolefin coverings and polyolefin films, which are being used increasingly instead of PVC films and floor coverings because of the disposal problems of halogenated products made of plastic.
Various routes are currently taken to overcome the wetting and gluing difficulties which occur with these polyolefin materials. For example, the use of solvent-containing primers (EP-A 0 539 710) or the development of adhesives based on modified rubbers (Polym. Prep., Jpn. Engl. Ed. 38(2) page E658, (1989)) has been mentioned. Other approaches lie in modification of the polyolefin materials to be glued by gas phase fluorination. In this case, the polymer surface is functionalized by reaction with fluorine. The current state of the art is that the desired gluing strengths can be achieved on polyolefins only by fluorination (Kleben und Dichten, 38(6), 1994, 21-24).
A solution for gluing untreated polyolefin coverings is urgently sought, specifically in the field of aqueous floor adhesives, since the action of the pretreatment cannot be retained over the storage period under the different storage conditions of the covering materials, and also any other modification requires an additional further working operation and therefore gives rise to costs.
Floor adhesives based on aqueous dispersions for gluing all the usual floor coverings, including parquet, have established themselves on the market. They are described in Patent Applications DE-A 23 01 497 and EP-A 0 221 461 and comprise an aqueous binder in the form of a plastics dispersion, resins, and fillers. The resin is generally built up from colophony, tall resins, or esters of the natural resins mentioned, and is used in organic, low-boiling solvents having a flash point below 55.degree. C. Such floor adhesives, which comprise 3 to 5% by weight of toluene or also xylene, are now regarded as outdated and can be employed under only limited conditions because of existing hazardous substances legislation.
Resin formulations, although they are regarded as solvent-free, generally comprise high-boiling solvents, such as butyldiglycol, butyldiglycol acetate, dibutyl phthalate or else high-boiling ethers, such as diethylene glycol monobutyl ether, or else polyethylene glycol dimethyl ether, ethylene glycol dimethyl ether or di- or triethylene glycol dimethyl ether, as well as polymeric plasticizers have preferably been used for modification of adhesives. U.S. Pat. No. 4,654,388 and DE-A 20 19 233 describe floor adhesives which comprise a film-forming vinyl acetate/ethylene/acrylate latex, filler, diethylene glycol monobutyl ether or dibutyl phthalate and a mixture of colophony resin and esters.
Adhesives which are free from natural resin and are based on vinyl acetate/ethylene can be improved by incorporation of functional comonomers. In GB-A 1 407 827 and EP-A 0 327 376, hydrolyzable silicon compounds are copolymerized for this purpose. EP-A 0 365 980 carries out the polymerization in the presence of 1 to 5% by weight of an olefinically unsaturated carboxylic acid. According to EP-A 0 216 210, the simultaneous presence of ethylenically unsaturated carboxylic acids and hydroxyalkyl-functional compounds is necessary.
An alternative process for the preparation of an improved adhesive based on vinyl acetate/ethylene proposes the use of certain dispersing agents. In DE-A 27 18 716, vinyl acetate and ethylene are polymerized in the presence of 1 to 8% by weight of a nonionic surfactant of the polyglycol ether type and 0.1 to 1% by weight of a water-soluble high molecular weight protective colloid for this purpose. EP-A 0 295 727 also proposes polymerization in the presence of customary emulsifiers and protective colloids. The protective colloids are added in an amount of 0.02 to 2% by weight, based on the emulsion, corresponding to 0.005 to 1.3% by weight, based on the monomers. Small amounts of cellulose ether are used as protective colloids in these dispersions based on vinyl acetate and ethylene, in order to increase the stability and to be able to prepare fine-particle dispersions. The protective colloid component is employed in the range up to 1.3% by weight here. Adhesives prepared in this manner show an outstanding adhesive force, for example on polyvinyl chloride, polystyrene and polyester materials, but not on untreated, non-modified polyolefin materials (Comparison Example 2).
The floor adhesives prepared with the known customary plastics dispersions and adhesive recipes are not capable of gluing coverings based on polyolefins with adequate strength. The copolymer dispersions according to the present invention have been developed in order also to be able to meet the conditions required on untreated polyolefin coverings.